The present invention relates to a printing ink containing microcapsules which has the consistency of a paste and is suitable for use in the manufacture of business forms by application from the ink fountain tray of a dry offset printing tower.
In pressure-sensitive recording papers, more commonly referred to as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor (the color former) is coated on the back side of the front sheet of paper in a form set. This backside coating is known as the CB coating and the sheet carrying it is known as the CB sheet. In order to form an image the CB coating is mated with a paper coated with a suitable color developer on its front side. This coating is known as a CF coating and the sheet is known as the CF sheet. Marks are made by rupturing the capsules in the CB coating by pressure to cause the color former to release from the capsules and react with the developer whereupon the color former is converted to its colored form and the mark is formed.
The Mead Corporation pioneered the development of compositions containing microcapsules which are suitable for on press application in the manufacture of business forms. These compositions are characterized by their ability to set rapidly, with minimal drying, so that they can be applied to the forms at the printing speeds normally encountered on a forms press. One of the compositions which Mead continues to commercialize is a hot melt composition in which the microcapsules are incorporated into a coating composition containing a wax binder. Other printing ink compositions have also been developed which are aqueous based compositions but which contain minimal amounts of water so that they can meet the drying speed requirements for application on a forms press. One such composition is described in U.S. Pat. No. 4,889,877 to Seitz and utilizes a slurry of microcapsules which is prepared by microencapsulating the color former in situ in a printing ink vehicle. Another composition described in International Patent Application WO 95/02643 is prepared by spray drying the microcapsules and adding the spray dried capsules to a printing ink vehicle. The compositions which have been developed to date have been designed for application from the dampening system tray but they are not suitable for application from the ink fountain tray of a dry offset printing tower because they are not viscous enough. The printing ink compositions applied from the ink fountain tray must have the consistency of a paste.